Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade‐wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large‐eddy simulations for an idealized case of trade‐wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade‐wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade‐wind layer.

中文翻译：

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风切变如何影响顺风积云对流

受观测到的海洋低云量覆盖与地表风速之间关系的启发，本研究调查了垂直风切变如何影响商风积云对流，包括浅积云和顶部低于冰点的充血。我们使用在纬向风中使用不同垂直剪切力的理想风向对流情况进行了大涡模拟。向后剪切使地表东风成为上西风，有效地限制了垂直云的发展，从而导致了较湿的，较浅的和较多云的贸易风层。在没有剪切力或没有正向剪切力的情况下，浅对流往往会加深，但是在正向剪切力作用下云顶仍然受到限制。多种机制解释了观察到的行为：首先，剪切力导致不同的表面风速，进而，由于动量传递而产生的表面热量和水分通量，从而在逆向剪切作用下产生了最弱的表面风速。其次，亚云层中的正向剪切剖面会增强水分聚集并导致更大的云团簇，但只能在通常支持云组织的大型域上进行。第三，穿过云层的任何绝对剪切力都会通过增加向下的压力扰动力来限制上升气流的速度。因此，可以认为反向剪切是冬季贸易中最典型的剪切，它是设定贸易风层典型结构的关键因素。但仅在通常支持云组织的大型域上。第三，穿过云层的任何绝对剪切力都会通过增加向下的压力扰动力来限制上升气流的速度。因此，可以认为反向剪切是冬季贸易中最典型的剪切，它是设定贸易风层典型结构的关键因素。但仅在通常支持云组织的大型域上。第三，穿过云层的任何绝对剪切力都会通过增加向下的压力扰动力来限制上升气流的速度。因此，可以认为反向剪切是冬季贸易中最典型的剪切，它是设定贸易风层典型结构的关键因素。